1. Field of Invention
The present invention relates to cellular communication systems; and more particularly, to a method and system for effecting a handoff of an on-going cellular communication from a serving cell to a neighboring cell of a cellular communication system using signal strength measurements from mobile units having scanning capability.
2. Description of Related Art
A digital cellular communication system includes several base transceiver stations, each of which transmits and receives channels of RF information throughout a predetermined coverage area referred to as a cell. The outer boundary of a cell is determined not only by the effective radiated power of the base transceiver, which determines the maximum range at which a mobile unit is able to receive; but also, by the effective radiated power of the mobile telephones, which determines the maximum range at which the base station is able to receive.
In current cellular communication systems of the digital type, each mobile unit has scanning capability for control and/or scanning traffic channels in neighboring cells that may or may not overlap the cell in which the mobile unit is controlled, herein referred to as a serving cell. The scanning capability of the mobile units may be either controlled by commands from base station controller (BSC) and/or commence scanning upon detecting a received signal strength below a particular value.
Typically, a mobile unit would scan pre-selected control and/or traffic channels of the neighboring cells and report signal strength to corresponding base station controllers. When the signal strength equalled or exceeded predefined threshold values configured in the base station, the system would switch an on-going communication to the appropriate traffic channel of the neighboring cell, thus, effecting a handoff. This method requires that the digital mobile unit is able to receive the neighboring cells transmission, but does not require that the base transceiver station is able to receive the mobile units transmission.
This method for effecting handoff is quite satisfactory for high power mobiles. However, in the case of low power mobiles in large cells, handoff based solely on the raw mobile measurements results in dropped calls, because the mobile, although receiving a strong signal from the base station, is too far away for adequate reception at the base station. One prior art arrangement for overcoming this problem involves using the neighboring base station scanning receiver measurement to confirm the mobile measurements.
In view of the increasing use of low power digital units for cellular communication, and in light of the foregoing, there is a need for a system and method for effecting handoffs for digital mobile units that overcome the disadvantages of prior arrangements without the use of costly base station scanning receivers.